Thank you for posting; my apologies for the late response.
To assist you, more information is necessary. Since the error is generated by the Windows operating system, I suggest you ensure that you are not experiencing other operating system issues. If you had successfully installed and were using FileMaker Pro 6, you may try rebooting your machine, and opening the file from the File menu, just to be sure you locate the actual FileMaker file on your hard drive. Have you tried to open any other files, or tried to create a new database file? More detalis about what you're doing when the error appears would be helpful.
I realize it has been a month since your post, so if you have resolved your issue, please post here so that others may benefit.